Several systems have been proposed for transmitting stereo program material on the amplitude modulated (AM) carriers in the AM band, i.e., 540-1650 kilocycles. For example, see the proposal entitled "In The Matter of AM Stereophonic Broadcasting," docket No. 21313, submitted by Magnavox Consumer Electronics Company to the Federal Communications Commission.
In the Magnavox system, the left (L) channel audio signal and the right (R) channel audio signal are summed together to form an L+R signal. The L+R signal is used to amplitude modulate the radio frequency (RF) carrier signal in the normal manner. A difference signal, L-R, is used to phase modulate (PM) the RF carrier signal. The presence of stereo modulation is indicated by frequency modulating (FM) the RF carrier signal with a subsonic 5 hertz signal.
At the receiver, the L+R signal is recovered by an amplitude detector. The L-R signal is recovered by a phase detector. The L channel is separated from the R channel by summing the L+R signal with the L-R signal in order to cancel the R signal component. The R channel is separated from the L channel by subtracting the L-R signal from the L+R signal in order to cancel the L signal component.
The degree of separation between the left and right channels is dependent on the equality of the amplitudes of the demodulated L+R and L-R signals. That is, to the extent that the amplitude of the L+R signal is not the same as the amplitude of the L-R signal, the respective cancellation of L and R signal components will not be accurate, leaving a portion of the R signal in the separated L channel, and vice versa.
The amplitude of the demodulated L-R signal is relatively unaffected by the RF signal strength at the reciever. This is so because the amplitude of the L-R signal depends upon the phase and not the amplitude of the received signal. On the other hand, since the RF carrier is amplitude modulated in accordance with the L+R signal, the amplitude of the demodulated L+R signal is directly related to the RF signal strength at the receiver.
An automatic gain control (AGC) system may be used in the receiver to maintain the amplitude of the demodulated L+R signal at a relatively constant level regardless of the received RF signal strength. For purposes of channel separation, the AGC system should provide a nearly constant output, i.e., a relatively flat characteristic. An AGC system having a relatively flat characteristic is called a "hard" AGC system. A hard AGC system creates a problem in tuning a desired station. During tuning, when a station is first encountered, a hard AGC system provides full volume before the user accurately tunes the receiver to the station frequency. If the signal strength at the receiver is strong enough, the received audio signal will be acceptable even though the receiver is slightly mistuned. However, if the received RF signal strength decreases, such as in an automobile radio in a moving vehicle, then the received audio signal will become unacceptable and the receiver require further tuning. Received RF signal strength may also vary due to signal fading, i.e., the signal strength decreases. Therefore, a hard AGC characteristic hampers proper receiver tuning.
For purposes of accurate tuning, it is desirable that the receiver AGC system provide a somewhat increasing output for increasing signal input, i.e., a "soft" AGC characteristic. Then, the user can accurately tune the receiver by tuning for maximum volume. However, a soft AGC characteristic results in a demodulated L+R signal amplitude that is proportional to RF signal strength, which, for reasons explained above, reduces stereo channel separation.